Active brazing solder for brazing alumina-ceramic parts

ABSTRACT

The active brazing solder for brazing ceramic parts of alumina, particularly of high-purity alumina, contains a maximum of 12 wt. % Ti, a maximum of 8 wt. % Be, and less than 16.5 wt. % Fe, the remainder being Zr and any impurities that may be present. The active brazing solder has the following behaviour/features: Brazing temperature: lower than 1,000° C.; the brazed joint is high-vacuum-tight over a long period of time; the coefficient of thermal expansion of the active brazing alloy is substantially identical to that of the alumina ceramic in the entire temperature range covered during the brazing process; the strength of the brazed joint between the two ceramic parts is so high that under tensile loading, fracture will result not at the joint, but in the adjacent ceramic; the pressure resistance of the active brazing solder is greater than 2 GPa; the active brazing solder is very good processable into powders having particle sizes on the order of 10 μm.

This is a division of application Ser. No. 09/455,690, filed Dec. 7,1999 now U.S. Pat. No. 6,427,900, which is a continuation of applicationSer. No. 08/896,481 filed Jul. 18, 1997 now U.S. Pat. No. 6,039,918,which claims benefit of U.S. provisional application No. 60/023,079filed Aug. 2, 1996.

FIELD OF THE INVENTION

The present invention relates to active brazing solders for brazingceramic parts of alumina, particularly of high-purity alumina.

BACKGROUND OF THE INVENTION

Active brazing solders are alloys which contain at least one elementhaving an affinity for oxygen, such as titanium. They attack thecovalent or ionic bonding of the ceramic surfaces to be brazed, wetthese surfaces, and thus unite chemically and mechanically with them.Therefore, brazing requires no fluxes whatsoever.

Part of these active brazing solders, if they are brittle and difficultto machine or unmachinable in the solidified condition, can be producedby melt-spinning in the form of thin strips, which can then be easilymachined, e.g., stamped or cut.

Thus, shaped active brazing foil parts, such as rings, can be produced,which are placed between the ceramic parts to be brazed and aresubsequently fused with the latter.

Part of the molten and solidified active brazing alloys may also beground into powder and processed in this form into an active brazingpaste, which can also be introduced between the ceramic parts, e.g., inthe form of a ring, and subsequently fused with these parts.

When brazing ceramic parts of alumina, particularly of 99.9 percent,i.e., high-purity, alumina as is needed and used for capacitive orresistive ceramic pressure sensors, particularly absolute-pressuresensors, the active brazing solder must meet several requirements; inparticular, it must have the following properties:

The temperature at which the sintered alumina ceramic is brazed, i.e.,the brazing temperature, must be below 1000° C., preferably between 700°C. and 980° C.

The brazed joint must be high-vacuum-tight over a long period of time,so that a vacuum existing during the brazing process in the chamber of apressure sensor, for example, which is closed by the brazing, willremain unchanged.

The coefficient of thermal expansion of the active brazing alloy shouldbe identical to that of the alumina ceramic in the entire temperaturerange covered during the brazing process, so that only minimal stresswill be developed during cooling from the brazing temperature to theambient temperature.

The strength of the brazed joint between the two ceramic parts must beso high that under tensile loading, fracture will result not at thejoint, but in the adjacent ceramic.

The pressure resistance of the active brazing solder must be at least 2GPa (=2 Gigapascals).

An active brazing solder which meets these requirements should also beprocessable into the aforementioned active brazing pastes, since themelt-spinning process, if applicable, requires costly and complicatedequipment, so that the active brazing foils produced therewith areexpensive.

With active brazing solders such as the zirconium-nickel-titanium alloysdescribed in U.S. Pat. No. 5,351,938 (in the following abbreviated, asusual, as ZrNiTi alloys) not all of the above-mentioned boundaryconditions can be fulfilled in a completely satisfactory manner. Inparticular, the above-mentioned requirement that the coefficients ofthermal expansion of the active brazing solder and the alumina should beidentical over the entire temperature range is not met, this requirementbeing based on new knowledge gained by the inventors.

SUMMARY OF THE INVENTION

It was therefore necessary, and this is the problem underlying theinvention, to look for compositions of active brazing solders which aredifferent from those of the prior art zirconium-nickel-titanium alloys.

The invention provides an active brazing solder for brazingalumina-ceramic parts which contains a maximum of 12 wt. % titanium, amaximum of 8 wt. % beryllium, and less than 16.5 wt. % iron, theremainder being zirconium and any impurities that may be present.

In one preferred embodiment of the invention, the active brazing soldercontains 8.6 wt. % titanium, 4 wt. % beryllium, and 15.8 wt. % iron.

In another preferred embodiment, the active brazing solder contains 8.8wt. % titanium, 2 wt. % beryllium, and 16.2 wt. % iron.

In a further preferred embodiment, the active brazing solder contains8.9 wt. % titanium, 1 wt. % beryllium, and 16.3 wt. % iron.

In still another preferred embodiment of the invention, the activebrazing solder contains 10 wt. % titanium and 4 wt. % beryllium, but noiron.

An essential advantage of the active brazing solders according to theinvention is that they can be ground finely with a higher yield than theZrNiTi alloys described in the above-mentioned U.S. Pat. No. 5,351,938using equipment of comparable complexity, and that the grinding underoxygen described in U.S. Pat. No. 5,431,744 can be used.

In the oxygen atmosphere, the melted, cooled, and uncrushed pieces ofthe active brazing alloys of the invention begin to disintegrate into ahydride powder of the alloy (particle diameter of the order of less than300 μm) between 100° C. and 150° C. already at an absolute pressure ofapproximately 200 kPa (=200 kilopascals=2 bars). In a mill, e.g., a ballmill, this powder can be ground, under hydrogen overpressure and withlittle expenditure of energy, into powders with a desired mean particlesize on the order of 10 μm, e.g., 12 μm. The hydrogen can be removedlater during the brazing process.

The entire powder production process, namely hydrogenating, grinding,and screening, takes place in the absence of atmospheric oxygen.Grinding, storing, and packaging are carried out under hydrogen orinert-gas overpressure, so that air has no access. This ensures a lowoxygen content in the powders, which have a high affinity for oxygen, sothat one of the requirements for good brazing properties is met.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and further advantages will now be explained in moredetail with reference to the accompanying drawing, in which:

FIG. 1 is a plot of the expansion coefficients of high-purity aluminaand some active brazing solders according to the invention;

FIG. 2 shows various properties of preferred active brazing soldersaccording to the invention in the form of a table; and

FIG. 3 is a plot representing the effect of the iron content on theexpansion coefficient of a zirconium-iron-titanium alloy.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, measured values of the expansion coefficients a ofhigh-purity alumina, i.e., 99.9% alumina, and of some active brazingsolders according to the invention are plotted as a function oftemperature in the range between approximately 100° C. and approximately600° C. The ordinate represents the expansion coefficient α in 10⁻⁶/K,and the abscissa represents the temperature T in ° C.

Curve 1 shows the coefficient of thermal expansion of high-purityalumina. It runs between approximately 6·10⁻⁶/K at approximately 100° C.and approximately 9·10⁻⁶/K at approximately 550° C. with a slightlynegative curvature.

Curve 2 shows the coefficient of thermal expansion of azirconium-titantium-beryllium active brazing alloy with the compositionZrTi10Be4. This short notation means that the alloy contains 10 wt. %titanium (abbreviated: Ti), 4 wt. % beryllium (abbreviated: Be), and 86wt. % zirconium (abbreviated: Zr).

Curve 2 runs between approximately 7·10⁻⁶/K at approximately 100° C. andapproximately 9·10⁻⁶/K at approximately 550° C., with a slight maximumof approximately 9.2·10⁻⁶/K at approximately 450° C.

Curve 3 shows the coefficient of thermal expansion of thezirconium-iron-titanium-beryllium active brazing alloy with thecomposition (ZrFe16.5Ti9)99Bel; this means that the alloy contains 1 wt.% beryllium and 99 wt. % of a constituent composed of 16.5 wt. % iron(abbreviated: Fe), 9 wt. % titanium, and 74.5 wt. % zirconium.

The expression in parentheses of (ZrFe16.5Ti9)99Bel can be multipliedout, so that it can also be written as: ZrFe16.33Ti8.9Bel, i.e., 1 wt. %beryllium, 16.33 wt. % iron, 8.9 wt. % titanium, and 73.77 wt. %zirconium.

Curve 3 runs between approximately 7.5·10⁻⁶/K at approximately 100° C.and approximately 9.2·10⁻⁶/K at approximately 450° C., with a slightmaximum of approximately 9.3·10⁻⁶/K at approximately 350° C.

Curve 4 shows the coefficient of the thermal expansion of azirconium-iron-titanium-beryllium active brazing alloy with thecomposition (ZrFe16.5Ti9)98Be2; this means that the alloy contains 2 wt.% beryllium and 98 wt. % of a constituent composed of 16.5 wt. % iron, 9wt. % titanium, and 74.5 wt. % zirconium, or, again multiplied out,ZrFe16.17Ti8.8Be2, i.e., 1 wt. % beryllium, 16.17 wt. % iron, 8.8 wt. %titanium, and 73.03 wt. % zirconium.

Curve 4 runs between approximately 7·10⁻⁶/K at approximately 100° C. andapproximately 9.7·10⁻⁶/K at approximately 350° C.

FIG. 1 shows that the expansion coefficient of the active brazing alloyZrTi10Be4, which is represented by curve 2, comes closest to theexpansion coefficient of high-purity alumina, and that in thetemperature range shown, the expansion coefficient of the active brazingalloy differs from that of alumina by a substantially constant amount,which is only approximately +0.8·10⁻⁶/K.

According to the table of FIG. 2, however, at a brazing temperatureT_(L) of 940° C., this active brazing alloy ZrTi10Be4 has a tensilestrength R_(M) of approximately 35 MPa (=35 Megapascals), which isadequate in certain cases, but appears not yet suitable for the widerange of possible applications.

The highest value of the tensile strength R_(M) together with the lowestvalue of the brazing temperature T_(L) is achieved with the activebrazing alloy (ZrFe16.5Ti9)98Be2 according to curve 4 of FIG. 1, namelya value of R_(M)≈116 MPa and a value of T_(L)=920° C.

In the temperature range in which pressure sensors are commonly used,the coefficient of thermal expansion of the active brazing alloy(ZrFe16.6Ti9)98Be2 is nearly equal to that of the active brazing alloyZrTi10Be4.

To confirm a presumption on the part of the inventors that theaforementioned approximation of the coefficient of thermal expansion ato that of alumina is due to the iron content of the active brazingalloys, FIG. 3 shows the result of measurements made to determine thiseffect of iron. Zirconium-iron-titanium alloys of the compositionZrFe_(X)Ti10 were chosen, where the subscript x indicates the varyingiron content during the measurements, again in wt. %.

Curve 5 of FIG. 3 shows the respective coefficient of thermal expansionbetween 50° C. and 200° C., and curve 6 shows the respective coefficientof expansion between 100° C. and 200° C. From the virtually constantslopes of curves 5 and 6 the inventors have drawn the conclusion that,if the iron content x becomes vanishingly small, the expansioncoefficient α is smallest, namely approximately 6.3·10⁻⁶/K.

This probably accounts for the fact that the values of the expansioncoefficient of the iron-free alloy ZrTi10Be4 according to curve 2 ofFIG. 1 are lower than those of the iron-containing alloys of curves 3and 4 of FIG. 1.

What is claimed is:
 1. A capacitive or resistive pressure sensor,comprising: a first alumina-ceramic part; a second alumina-ceramic part;and a brazed joint that unites said first alumina-ceramic part to saidsecond alumina-ceramic part, said brazed joint formed from a brazingsolder consisting essentially of 8.6 wt. % to 12 wt. % titanium, 1 wt. %to 8 wt. % beryllium, 0 wt. % to 16.5 wt. % iron, 63.5 wt. % to 90.4 wt.% zirconium, and a remainder being any impurities.
 2. The pressuresensor of claim 1, wherein said active brazing solder has a meltingpoint below 1000° C.
 3. The pressure sensor of claim 1, wherein saidactive brazing solder has a melting point between 700° C. and 980° C. 4.The pressure sensor of claim 1, wherein said brazed joint was formedfrom an active brazing paste comprising said active brazing solder. 5.The pressure sensor of claim 1, wherein said active brazing solderconsists essentially of about 10 wt. % titanium, about 4 wt. %beryllium, and about 86 wt. % zirconium.
 6. The pressure sensor of claim1, wherein said active brazing solder consists essentially of about 8.6wt. % titanium, about 4 wt. % beryllium, about 15.8 wt. % iron, andabout 71.6 wt. % zirconium.
 7. The pressure sensor of claim 1, whereinsaid active brazing solder consists essentially of about 8.8 wt. %titanium, about 2 wt. % beryllium, about 16.2 wt. % iron, and about 73wt. % zirconium.
 8. The pressure sensor of claim 1, wherein said activebrazing solder consists essentially of about 8.9 wt. % titanium, about 1wt. % beryllium, about 16.3 wt. % iron, and about 73.8 wt. % zirconium.9. The pressure sensor of claim 1, wherein said brazed joint has apressure resistance of at least 2 Gigapascals.
 10. The pressure sensorof claim 1, wherein said first ceramic part, said second ceramic part,and said brazed joint define a vacuum-tight chamber.